This invention relates to a method and apparatus for continuously cooling a moving web, and particularly to a method and apparatus for strip casting of metals in which an endless belt is cooled in a manner to improve the quality of the metal cast.
The continuous casting of thin metal strip has been employed with only limited success. By and large, prior processes for the continuous casting of metal strip have been limited to a relatively small number of alloys and products. It has been found that as the alloy content of various metals are increased, as-cast surface quality deteriorates. As a result, many alloys must be fabricated using ingot methods.
In the case of aluminum, relatively pure aluminum product such as foil can be continuously strip cast on a commercial basis. Building products can likewise be continuously strip cast, principally because surface quality in the case of such building products is less critical than in other aluminum products, such as can stock. However, as the alloy content of aluminum is increased, surface quality problems appear, and strip casting has generally been unsuitable for use in making many aluminum alloy products.
A number of strip casting machines have been proposed in the prior art. One conventional device is a twin belt strip casting machine, but such machines have not achieved widespread acceptance in the casting of many metals, and particularly metal alloys with wide freezing ranges. In such twin belt strip casting equipment, two moving belts are provided which define between them a moving mold for the metal to be cast. Cooling of the belts is typically effected by contacting a cooling fluid with the side of the belt opposite the side in contact with the molten metal. As a result, the belt is subjected to extremely high thermal gradients, with molten metal in contact with the belt on one side and a water coolant, for example, in contact with the belt on the other side. The dynamically unstable thermal gradients cause distortion in the belt, and consequently neither the upper nor the lower belt is flat. The product thus produced has areas of segregation and porosity as described below.
Leone, in the Proceedings Of The Aluminum Association, Ingot and Continuous Casting Process Technology Seminar For Flat Rolled Products, Vol. II, May 10, 1989, said that severe problems develop if belt stability and reasonable heat flow are not achieved. In the first place, if any area of the belt distorts after solidification of the molten metal has begun and strip shell coherency has been reached, the resulting increase in the gap between the belt and the strip in the distorted region will cause strip shell reheating, or, at least, a locally reduced shell growth rate. That, in turn, gives rise to inverse segregation in the strip which generates interdendritic eutectic exudates at the surface. Moreover, in severe cases with medium and long freezing range alloys, liquid metal is drawn away from a distorted region to feed adjacent, faster solidifying portions of the strip. That in turn causes the surface of the strip to collapse and forms massive areas of shrinkage porosity in the strip which can crack on subsequent rolling or produce severe surface streaks on the rolled surface.
As a result, twin belt casting processes have not generally achieved acceptance in the casting of alloys for surface-critical applications, such as the manufacturing of can stock. Various improvements have been proposed in the prior art, including preheating of the belts as described in U.S. Pat. Nos. 3,937,270 and 4,002,197, continuously applied and removed parting layers as described in U.S. Pat. No. 3,795,269, moving endless side dams as described in U.S. Pat. No. 4,586,559 and improved belt cooling as described in U.S. Pat. Nos. 4,061,177, 4,061,178 and 4,193,440. None of those techniques has achieved widespread acceptance either.
Another continuous casting process that has been proposed in the prior art is that known as block casting. In that technique, a number of chilling blocks are mounted adjacent to each other on a pair of opposing tracks. Each set of chilling blocks rotates in the opposite direction to form therebetween a casting cavity into which a molten metal such as an aluminum alloy is introduced. The liquid metal in contact with the chilling blocks is cooled and solidified by the heat capacity of the chilling blocks themselves. Block casting thus differs both in concept and in execution from continuous belt casting. Block casting depends on the heat transfer which can be effected by the chilling blocks. Thus, heat is transferred from the molten metal to the chilling blocks in the casting section of the equipment and then extracted on the return loop. Block casters thus require precise dimensional control to prevent flash (i.e. transverse metal fins) caused by small gaps between the blocks. Such flash causes sliver defects when the strip is hot rolled. As a result, good surface quality is difficult to maintain. Examples of such block casting processes are set forth in U.S. Pat. Nos. 4,235,646 and 4,238,248.
Another technique which has been proposed in continuous strip casting is the single drum caster. In single drum casters, a supply of molten metal is delivered to the surface of a rotating drum, which is internally water cooled, and the molten metal is dragged onto the surface of the drum to form a thin strip of metal which is cooled on contact with the surface of the drum. The strip is frequently too thin for many applications, and the free surface has poor quality by reason of slow cooling and micro-shrinkage cracks. Various improvements in such drum casters have been proposed. For example, U.S. Pat. Nos. 4,793,400 and 4,945,974 suggest grooving of the drums to improve surface quality; U.S. Pat. No. 4,934,443 recommends a metal oxide on the drum surface to improve surface quality. Various other techniques are proposed in U.S. Pat. Nos. 4,979,557, 4,828,012, 4,940,077 and 4,955,429.
Another approach which has been employed in the prior art has been the use of twin drum casters, such as in U.S. Pat. Nos. 3,790,216, 4,054,173, 4,303,181, or 4,751,958. Such devices include a source of molten metal supplied to the space between a pair of counter-rotating, internally cooled drums. The twin drum casting approach differs from the other techniques described above in that the drums exert a compressive force on the solidified metal, and thus effect hot reduction of the alloy immediately after freezing. While twin drum casters have enjoyed the greatest extent of commercial utilization, they nonetheless suffer from serious disadvantages, not the least of which is an output typically ranging about 10% of that achieved in prior art devices described above. Once again, the twin drum casting approach, while providing acceptable surface quality in the casting of high purity aluminum (e.g. foil), suffers from poor surface quality when used in the casting of aluminum with high alloy content and wide freezing range. Another problem encountered in the use of twin drum casters is center-line segregation of the alloy due to deformation during solidification.
In co-pending application Ser. No. 07/902,997 filed Jun. 23, 1992, there is described an improved method and apparatus for the continuous casting of metals, and particularly aluminum, utilizing at least one endless belt or web formed of a heat conductive material. In the method and apparatus described in that co-pending application, the disclosure of which is incorporated herein by reference, a molten metal such as aluminum is deposited on the surface of the belt to form a cast metal strip. The sensible heat of fusion of the metal is removed from the belt by means of cooling the belt when the belt is not in contact with the metal. As is described in the aforementioned co-pending application, the method and apparatus there disclosed utilizes the heat sink capacity of the belt in casting the molten metal to minimize or avoid erratic distortion effects caused by high non-uniform thermal gradients across the belt. The present invention is an improvement over the method and apparatus described in the foregoing co-pending application, and is directed to a method and apparatus for continuously cooling the endless belt in which the cooling or quenching fluid is contained so as not to contaminate the remainder of the casting process, or without the need to use complex and expensive contacting seals.
It is accordingly an object of the invention to provide a method and apparatus for continuously cooling an endless belt by means of a cooling fluid in which the temperature of the belt is accurately controlled, and, at the same time, is contained without contamination of the adjacent processes without the need to employ complex and costly seals. The casting process requires that the heat transferred from the product is extracted by quenching the belts in a controlled manner. The belt temperature at the point where molten metal is introduced must be accurately controlled because it is critical to the process, affecting the thickness of products. It is also important to the process and to product surface quality that the temperature profile over the width is controlled in incremental zones in order to maintain belt flatness and affect uniformity of thermal contact between belts and products.
The quench system when applied for quenching the product requires similar attributes of zone controlled quenching rates in order to provide successful, uniform metallurgical processing; i.e., to retain the elements in solid solution, thereby increasing strength and improving corrosion resistance.
In both applications, the quenching media must be entirely contained within the quenching devices. In the case of belt quenching, it is imperative that no trace of quenching media is allowed to enter the region of molten metal introduction for reasons of surface quality and safety. When quenching products, the finished strip must be free of moisture in order to prevent water stain.
These and other objects and advantages of the invention appear more fully hereinafter from a detailed description of the invention.